Final answer:
A fly with an X:A ratio of 0.5 and homozygous for a nonsense dsx allele is expected to be male, as the X:A ratio determines male development and the dsx gene typically would not change this outcome.
Step-by-step explanation:
The phenotype of a fly with an X:A ratio of 0.5 and that is homozygous for a dsx allele with a nonsense allele in exon 4 would be expected to be male. The doublesex (dsx) gene is known to be involved in sex determination in Drosophila, and flies with an X:A ratio of 0.5 typically develop as males. A nonsense allele in exon 4 could presumably truncate the dsx protein, potentially affecting its function. However, since the default sexual development pathway in flies without a functional dsx gene is male, it would not switch the expected male development triggered by the 0.5 X:A ratio.